Dancing With the Stars fans were shocked to find out the fan-favorite pro would be sitting out of his second performance with UFC fighter Paige VanZant on Monday night after injuring his back during a rehearsal. "I think I just slipped a disc," Mark said in the footage from the rehearsal shown during the show, with the dancer later saying he had an MRI earlier in the day.

And after Monday's show, Mark offered E! News' Sibley Scoles an update on his injury (one he previously attempted to perform through in a past season).

"The doctor seems to be optimistic, so I'm feeling good about it," he said, but said it's a matter of timing with an injury like this. "I just don't know how quickly. No time-frame yet...but I don't want to let her down. I want to be there with her, she's fantastic."

DWTS troupe member Alan Bernsten stepped in for Mark at the last minute, netting only two hours of practice with the MMA fighter. Despite the setbacks, Paige and Alan scored a 24/30 for their salsa, the highest score of the night. "I'm really proud of her," Mark told us. "I thought she did excellent tonight. It's a hard situation to be put into."

"It was scary. It was very scary," Paige admitted of the situation. "Allan stepped up and learned the routine very quickly. We had to change a lot of things, which was scary for me." And all day leading up to the live show, Paige was unsure of who would she dancing with, saying, "Everything was up in the air...I'm really happy I got to go out there and perform."

Now the question is just how serious Mark's injury is.

Like he told co-host Erin Andrews during the show, Mark told us he's set to get a CT scan and a bone scan "just to be thorough."

Paige added, "Hopefully he just gets better really soon!"

To hear more from Mark and Paige on his injury and her least favorite part of the competition (spoiler alert: it's the heels!), watch our interview with them above.